The elements Rb (Rubidium) and Sr (Strontium) are in the 5th period of the periodic table. This means that their outermost electrons are being filled into the 5s sublevel.

Here's why:

* Electron Configuration: The electron configuration of Rb is [Kr] 5s¹ and Sr is [Kr] 5s². Both elements have their last electron entering the 5s sublevel.

* Periodic Trends: Elements in the same period (horizontal row) have the same highest energy level. Since Rb and Sr are in the 5th period, their highest energy level is n=5. The s sublevel is the first sublevel to be filled in each energy level.

Let me know if you'd like to know more about electron configuration or periodic trends!